You misunderstand. Pension credit means testing is INCREDIBLY difficult and inefficient. It is not fit for purpose when it leaves 800,000 of the very poorest people. eligible to receive it, not getting it.
Tying WFA to it, just makes the situation even worse.
Then why are you talking about an extra £200 for a few rich people? You're not talking about making pension credit universal I assume?
Do you actually know the cost of administering pension credit? Given that it can pay out around £17k for a couple, then it's clearly going to be highly cost effective for it to be means tested.
And do you really know that the 800,000 are the very poorest? It's almost certain the majority either have other sources of income, are being supported in another way (e.g. are living with, or supported by close family), or are close enough to the limit that they don't believe it's worth applying.
No means tested benefit takeup will ever get close to 100%, and 14 years of the Tories has chipped away at a lot of the services that reach people who aren't claiming, but even so, most are still likely to have a reason for not applying.